Sports lovers grow up identifying strongly--like, insanely strongly--with certain players and teams. It's a huge part of their sense of self. It may not seem like a big deal, but if you make jabs at them, then you are basically ripping on him, too.

2. His Appearance When He Packs On A Couple Of Extra Pounds

Men deal with their love handles or beer belly by joking about it or pretending they never noticed they don't have six-pack abs. Truth is, guys are just as body-conscious as women are, so you'll hurt his feelings if you continually harp on him to eat less and work out more, making him even more resistant to following your advice.

3. His Loyalty To His Friends

Assuming his buddies aren't truly criminal or dangerous, don't rag on their flaws or ask him to stop hanging out with them. Even if your boyfriend knows you are right, he'll revert to guy code and defend them...and likely get pissed at you.